The present invention relates to a masking device for a colour cathode-ray display tube with a flat screen, of the type comprising a support frame for a tensioned shadowmask and a tensioned shadowmask mounted on the support frame.
Colour cathode-ray display tubes comprise, in a known manner, a display screen provided with phosphors, an electron gun producing 3 electron beams and a masking device consisting of a shadowmask mounted on a support frame, which is placed opposite the display screen and intended to ensure good quality of the image displayed. The shadowmask consists of a metal foil drilled with a plurality of holes or of slots through which the three electron beams pass in order to excite the phosphors placed on the screen. The quality of the image obtained is better the more precise the alignment between the phosphors, the holes in the shadowmask and the electron beams. When the display tube is in operation, a significant part of the electron beams is intercepted by the shadowmask, which results in the latter being heated locally, heating which can deform it and therefore degrade the quality of the image displayed. In addition, the quality of the image may also be degraded by the vibrations of the shadowmask which are caused by various vibration sources. To obtain high-quality images, the shadowmask must, on the one hand, be insensitive to localized heating and, on the other hand, to have a vibration eigenfrequency high enough for the amplitude of these vibrations not to disturb the colour of the images by a misalignment of the electron beams, of the holes in the shadowmask and of the phosphors.
When the display screen is curved, the shadowmask has a shape which matches that of the screen and the problems of sensitivity to localized heating and of vibration are solved by making the shadowmask by drawing a sheet of an Fexe2x80x94Ni alloy having a very low expansion coefficient and filled with holes. The shadowmask is simply welded to a support frame which exerts no force on the shadowmask. The frame may therefore be light, which has advantages.
When the display screen is flat, the shadowmask may be an undrawn foil fastened, for example, by welding it to a precompressed support frame which then exerts a tension in the shadowmask. The shadowmask is then referred to as a xe2x80x9ctensionedxe2x80x9d shadowmask. The tension in the shadowmask is intended, on the one hand, to solve the problem of sensitivity to localized heating and, on the other hand, to increase the vibration eigenfrequency of the shadowmask in order to attenuate the amplitude of these vibrations. This solution assumes, in particular, the use of a material whose characteristics allow a sufficient tension to be maintained in the operating temperature range of the cathode-ray tube (approximately 100xc2x0 C.) , and this being so after heating to approximately 500xc2x0 C. during the manufacture of the cathode-ray tube. This is because the shadowmask is mounted tensioned on its support frame and then the assembly is placed in the cathode-ray tube which is then sealed at a temperature of approximately 500xc2x0 C. for one hour. This heating may cause both the shadowmask and its frame to creep, which may detension the shadowmask.
In order to manufacture a tensioned shadowmask and its support frame, it has been proposed to use a low-alloy steel (i.e. a steel generally containing less than 5% of alloying elements). However, since the thermal expansion coefficient of this steel is high, the tension in the shadowmask must be greater than 200 MPa in order to avoid deformations due to the localized heating. This solution leads to a heavy frame, the weight of which may be as much as 6 kg or even higher.
In order to manufacture a tensioned shadowmask and its support frame, it has also been proposed to produce the shadowmask from an Fexe2x80x94Ni alloy having a low expansion coefficient and the frame from steel. However, it is then necessary to provide means for preventing the shadowmask from being overtensioned when sealing the tube at 500xc2x0 C., otherwise the shadowmask will tear during this operation.
The object of the present invention is to remedy these drawbacks by proposing a means for manufacturing a tensioned shadowmask and its support frame which are insensitive to localized heating, have a suitable vibration eigenfrequency and are well able to withstand the operation of sealing the tube at high temperature.
For this purpose, the subject of the invention is a masking device for a colour cathode-ray display tube with a flat screen, of the type comprising a support frame for a tensioned shadowmask and a tensioned shadowmask mounted on the support frame so as to undergo tensioning at room temperature. The support frame is made of a hardened Fexe2x80x94Ni alloy having a thermal expansion coefficient between 20xc2x0 C. and 150xc2x0 C. of less than 5xc3x9710xe2x88x926 Kxe2x88x921 and a yield stress Rp0.2 at 20xc2x0 C. of greater than 700 MPa and the tensioned shadowmask is made of a hardened Fexe2x80x94Ni or FeNi alloy having a thermal expansion coefficient between 20xc2x0 C. and 150xc2x0 C. of less than 5xc3x9710xe2x88x926 Kxe2x88x921.
The hardened Fexe2x80x94Ni alloy of which the support frame is composed may, for example, be a structurally hardened Fexe2x80x94Ni alloy of the xe2x80x9cxcex3xe2x80x2-hardened xe2x80x9ctypexe2x80x9d whose chemical composition is such that (in percent by weight):
40.5%xe2x89xa6Ni+Co+Cuxe2x89xa644.5%
0%xe2x89xa6Coxe2x89xa65%
0%xe2x89xa6Cuxe2x89xa63%
1.5%xe2x89xa6Tixe2x89xa63.5%
0.05%xe2x89xa6Alxe2x89xa61%
Cxe2x89xa60.05%
xe2x80x83Sixe2x89xa60.5%
Mnxe2x89xa60.5%
Sxe2x89xa60.01%
Pxe2x89xa60.02%
the balance being iron and impurities resulting from the smelting.
The hardened Fexe2x80x94Ni alloy of which the support frame is composed may also be an Fexe2x80x94Ni alloy of the xe2x80x9ccarbide-hardenedxe2x80x9d type having a chemical composition such that (in percent by weight):
36%xe2x89xa6Ni+Co+Cuxe2x89xa640%
0%xe2x89xa6Coxe2x89xa65%
0%xe2x89xa6Cuxe2x89xa63%
1.6%xe2x89xa6Moxe2x89xa62.8%
0.4%xe2x89xa6Crxe2x89xa61.5%
0.15%xe2x89xa6Cxe2x89xa60.35%
Sixe2x89xa60.5%
Mnxe2x89xa60.5%
Sxe2x89xa60.01%
Pxe2x89xa60.02%
the balance being iron and impurities resulting from the smelting.
The hardened Fexe2x80x94Ni alloy of which the support frame is composed may also be an Fexe2x80x94Ni alloy of the xe2x80x9cberyllium-hardenedxe2x80x9d type having a chemical composition such that (in percent by weight):
34%xe2x89xa6Ni+Co+Cuxe2x89xa638%
0%xe2x89xa6Coxe2x89xa65%
0%xe2x89xa6Cuxe2x89xa63%
0.15%xe2x89xa6Bexe2x89xa61%
Cxe2x89xa60.05%
Sixe2x89xa60.5%
Mnxe2x89xa61%
Sxe2x89xa60.01%
Pxe2x89xa60.02%
the balance being iron and impurities resulting from the smelting.
The hardened Fexe2x80x94Ni alloy of which the support frame is composed may also be an Fexe2x80x94Ni alloy of the
xe2x80x9csolid-solution-hardenedxe2x80x9d type having a chemical composition such that (in percent by weight):
38%xe2x89xa6Ni+Co+Cuxe2x89xa642%
0%xe2x89xa6Coxe2x89xa65%
0%xe2x89xa6Cuxe2x89xa63%
1%xe2x89xa6Nbxe2x89xa64%
Cxe2x89xa60.05%
Sixe2x89xa60.5%
Mnxe2x89xa60.5%
Sxe2x89xa60.01%
Pxe2x89xa60.02%
the balance being iron and impurities resulting from the smelting.
Preferably, the shadowmask is an Fexe2x80x94Ni alloy whose thermal expansion coefficient between 20xc2x0 C. and 150xc2x0 C. is less than 2xc3x9710xe2x88x926 Kxe2x88x921 and whose chemical composition may comprise (in percent by weight):
ti 32%xe2x89xa6Nixe2x89xa637%
0%xe2x89xa6Coxe2x89xa65.5%
0%xe2x89xa6Mnxe2x89xa60.5%
Sixe2x89xa60.2%
Cxe2x89xa60.02%
Sxe2x89xa60.01%
Pxe2x89xa60.02%
the balance being iron and impurities resulting from the smelting. The tension in the shadowmask is then preferably less than 120 MPa.
The shadowmask may also be made of a hardened Fexe2x80x94Ni alloy of the xe2x80x9cxcex3xe2x80x2-hardenedxe2x80x9d type, of the xe2x80x9ccarbide-hardenedxe2x80x9d type, of the xe2x80x9cberyllium-hardenedxe2x80x9d type or of the xe2x80x9csolid-solution-hardenedxe2x80x9d type, as defined above. The tension in the shadowmask may then be greater than 150 MPa.
The invention also relates to a process for the manufacture of the shadowmask support frame of a masking device for a colour cathode-ray display tube with a flat screen, the shadowmask support frame of which is made of a xe2x80x9cxcex3xe2x80x2-hardenedxe2x80x9d Fexe2x80x94Ni alloy. According to this process, a strip of a xe2x80x9cxcex3xe2x80x2-hardenedxe2x80x9d Fexe2x80x94Ni alloy, which is annealed or annealed and work-hardened and then stress-relieved, is used, with which strip a frame blank is produced by cutting, bending and welding, and then the frame blank undergoes a hardening heat treatment at a temperature of between 600xc2x0 C. and 800xc2x0 C. for a time of between 30 minutes and 2 hours.
The invention also relates to a process for the manufacture of the shadowmask support frame of a masking device for a colour cathode-ray display tube with a flat screen, the shadowmask support frame of which is made of a xe2x80x9ccarbide-hardenedxe2x80x9d Fexe2x80x94Ni alloy. According to this process, the shadowmask support frame is manufactured by cutting, bending and welding a sheet of a xe2x80x9ccarbide-hardenedxe2x80x9d Fexe2x80x94Ni alloy, obtained by cold rolling with a reduction ratio of greater than 50%, and by a hardening heat treatment at a temperature of between 650xc2x0 C. and 850xc2x0 C. for 1 minute to 2 hours, optionally followed by a complementary cold-rolling step with a reduction ratio of less than 70% and by a stress-relieving heat treatment at a temperature of between 400xc2x0 C. and 600xc2x0 C.
When the Fexe2x80x94Ni alloy is of the xe2x80x9cberyllium-hardenedxe2x80x9d type, the cold rolling is carried out with a reduction ratio of between 20% and 80% and the hardening treatment is a soak between 400xc2x0 C. and 700xc2x0 C. for a time of between 1 minute and 8 hours.
When the Fexe2x80x94Ni alloy is of the xe2x80x9csolid-solution-hardenedxe2x80x9d type, the cold rolling is carried out with a reduction ratio of between 20% and 70% and the heat treatment is a stress-relieving treatment corresponding to a soak between 400xc2x0 C. and 600xc2x0 C.
It should be noted that, instead of being formed by cutting and bending a strip, the frame may be manufactured by assembling tubes of square, triangular or round cross section. The hardening heat treatment is carried out either before or after the frame is mounted.